8 METABOLIC PATHWAYS IN MICROORGANISMS 



its pH optimum at 8.6. We have purified this enzyme 

 about 100-fold and have found it to be strictly TPN-specific. 

 The reaction is reversible, as tested with 8-D-gluconolactone 

 and TPNH, but the y-lactone is less active. Among the 

 sugars and their phosphates tested, only D-glucose and 2- 

 deoxy-D-glucose were oxidized, whereas glucose-6-phosphate, 

 glucose- 1 -phosphate, gluconolactone, and gluconic acid 

 were not attacked. This enzyme therefore appears different 

 from those described by de Ley and Stouthamer (12) for 

 this organism in the oxidation of gluconate. 



Glucose-6-Phosphate and 6-P ho spho gluconate. These 

 two soluble enzymes usually are closely associated in A. 

 suhoxydans, as they are in most organisms. They have 

 been highly purified, to the point that they catalyze the dis- 

 appearance of about 100 and 50 /xmoles of substrate per 

 minute, respectively, per milligram of enzyme. With a pH 

 optimum at 8.0, the turnover numbers for G-6-P dehydro- 

 genase are about equal with DPN and TPN, whereas DPN 

 is much superior with 6-phosphogluconic dehydrogenase. 

 The purifications are effected by combinations of protamine 

 precipitation, ammonium sulfate fractionation, calcium 

 phosphate gel adsorption, and column electrophoresis (13). 

 A comparison of properties of G-6-P dehydrogenase from 

 various sources is given in Table 1.2, whereas Table 1.3 

 contains a similar listing of properties of 6-PGA dehydro- 

 genases. 



For clarification, the enzymes that oxidize glucose or glu- 

 cose-6-phosphate, as reported from different laboratories, 

 are listed in Table 1.4 along with some of their distinguish- 

 ing properties. It is evident from Table 1.4 that A. 

 suboxydans is well endowed with separate enzymes all of 

 which attack these substrates. The reasons for this diversity 

 are not yet clear. It is possible that there is little or no 



